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Does a Bathtub Need a P-Trap?

Does a Bathtub Need a P-Trap?

Rough-in plumbing for a bathtub can be tricky. You may have heard that installing a P-Trap is not really necessary. Let’s look at the facts.

Does a bathtub need a P-Trap? Yes, you should always install a P-Trap as part of the plumbing of a bathtub or any other wastewater drain system. Failure to include a P-Trap could result in foul smells, harmful gases, and small animals entering through your tub’s drain.

Let’s take a hard look at the purpose of a P-Trap, the issues you’ll likely face if you don’t install one, and some of the more common malfunctions that a P-trap can have.

Note: A P-Trap is inexpensive and fairly easy to install depending on your construction. You can buy them at your local hardware store or online (link to Amazon).

The point is, it is not worth skipping this step in the installation process. It will only lead to issues as we will explain below.

What is a P-Trap?

A P-Trap is a curved piece of plumbing pipe used under sinks, showers, toilets, and other water drainage fixtures. The downward curvature creates a section of pipe that traps water in it on purpose. This trapped water is called the trap seal.

Trap seal is measured from the crown weir (the highest point on the inside of the curve) and the top of the dip of the trap. This distance shows the maximum amount of vertical depth of liquid the P-trap will hold (source). 

P-Traps come in several styles, including one-piece and two-piece designs. They are used for wall entry connections to the plumbing line and are assembled with compression style fittings.

Every time wastewater from the fixture enters this trap, it flushes the old water and any solids out, while still leaving water in the P-Trap to replace it. This continuous process of flushing out and replacing the water serves several purposes. 

Why Do I Need To Install A P-Trap?

There are a few very important reasons to install a P-Trap under a bathtub installation. All of them are important to your health and safety. 

  1. Sanitation: The most obvious reason for a P-Trap is to keep wastewater and sewage odors from backing up into the plumbing fixture. 
  1. Prevent harmful gases: Sewage creates many harmful gases that can potentially reenter the bathroom through drain pipes if not properly installed with P-Traps. These gases include:
  • Methane
  • Carbon Monoxide
  • Nitrogen
  • Hydrogen Sulfide

Any of these gases could seriously affect your health. Methane can cause nausea, headache, suffocation, and loss of consciousness (source). Hydrogen Sulfide can cause asthmatic responses and has the characteristic rotten egg smell (source). Most people know carbon monoxide and nitrogen can silently kill you. 

However, the water in a P-Trap stops gases from entering the atmosphere of your bathroom, and therefore protects you from the effects of these gases. 

3.     Trap Solids: This may seem counter-intuitive, but P-Traps collect solids, like hair and lint. During the normal process of running water through the trap, the solids eventually break down and are flushed away. 

Of course, this can build up too much and clog the drain. P-Traps make clogged drains easier to clean by keeping the sediment buildup within easy reach. Chemical drain cleaners, plungers, hooked extraction devices (snakes), and other devices can clear clogs close the fixtures, like in the P-Trap (source).

And if you have ever lost a wedding ring down the sink or had a child shove a toy into the bathtub drain, you understand how important clearing the drain can be!

4.     Stop Vermin: P-Traps also serve the handy purpose of keeping mice, rats, and other vermin from traversing the entire drain system and entering your bathroom. 

Plumbing Codes 

Plumbing codes require P-Traps installed on all wastewater connections. A few specific international code requirements for bathtub installation are:

  • P-Traps must connect to the same size drain outlet, with a diameter of at least 1.5 inches
  • The vertical distance from the fixture to the trap can not exceed 24 inches
  • The horizontal distance from the fixture to the trap should not exceed 30 inches
  • Each trap seal should be at least 2 inches deep, but not more than 4 inches
  • Only one trap can be installed per drain

These codes ensure proper functioning of a P-Trap in a bathtub installation (source). 

My Tub Doesn’t Have A P-Trap

Many people assume their bathtub doesn’t have a p-trap because they can’t see it like they can with a sink. However, tub p-traps can often be found under the foundation. Since the tub sits so low to the ground, the p-trap is often pre-installed by a plumber in the concrete or between floor joists.

This video is covering the process process of installing an acrylic tub but I have it set to start where the wastewater drain system is being connected. This is a case where the p-trap is actually above the ground and I’m including it because it provides a visual point of reference.

Do P-Traps Wear Out or Malfunction?

A few problems can occur with P-Traps, causing them to malfunction or break down. Proper installation and ventilation of the plumbing line can mitigate many of these issues. 

  1. Siphonage: If the water in a trap is siphoned from the drain by other pipes’ suction power, the trap could be left dry and unable to function. This happens when there is enough negative pressure to create a siphon to draw water out of the trap. (source).
  1. Back Pressure: Conversely, wastewater can back up in a drainage system through positive pressure, causing the water to build up and blow past the P-Trap and into the fixture.
    Even if water does not back-flow out of the fixture, air bubbles can be forced up the drain, causing gases to enter the room. 
  1. Wind Effect: Sometimes strong winds can compromise the traps by creating small instances of siphonage and back pressure. You may have noticed this effect when the water level seems to rise and fall in a toilet.
  1. Evaporation: Perhaps the most common issue is evaporation caused by low use of the fixture. This can lead to an empty P-Trap, and therefore the possibility of sewer gases building up in the drain and room. If you have a drain in your home that you do not use on at least a weekly basis, the potential of P-Trap failure increases. 


P-Traps are an integral part of the plumbing system in a bathtub installation. P-Traps create a barrier between harmful sewage gases, odors, and buildup and your home.

They also make cleaning the drain easier. It is important to properly install traps and plumbing lines to avoid some common problems, but if installed correctly and used often, P-Traps should function well.

And finally, plumbing codes require P-Traps installed on all wastewater lines. 

You may also be interested in these bathroom-related articles:

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